Update bindEnvs() to add support for binding nested fields of the
Options struct to environment variables. The variable names are formed
by joining the nested fields' mapstructure tags with underscores (after
first converting to uppercase).

This is in preparation for adding a new nested struct for downstream
mTLS settings that will look something like this:

  downstream_mtls:
    ca_file: /path/to/CA/cert.pem
    enforcement: reject_connection

With this change, these fields would be bound to the variables
DOWNSTREAM_MTLS_CA_FILE and DOWNSTREAM_MTLS_ENFORCEMENT.

Move downstream mTLS settings to a nested config file object, under the
key 'downstream_mtls', and add a new DownstreamMTLSSettings struct for
these settings.

Deprecate the existing ClientCA and ClientCAFile fields in the Options
struct, but continue to honor them for now (log a warning if either is
populated).

Delete the ClientCRL and ClientCRLFile fields entirely (in current
releases these cannot be set without causing an Envoy error, so this
should not be a breaking change).

Update the Settings proto to mirror this nested structure.

Add an "enforcement" option to the new downstream mTLS configuration
settings group.

When not set, or when set to "policy_default_deny", keep the current
behavior of adding an invalid_client_certificate rule to all policies.

When the enforcement mode is set to just "policy", remove the default
invalid_client_certificate rule that would be normally added.

When the enforcement mode is set to "reject_connection", configure the
Envoy listener with the require_client_certificate setting and remove
the ACCEPT_UNTRUSTED option.

Add a corresponding field to the Settings proto.

Add support for a new token $pomerium.client_cert_fingerprint in the
set_request_headers option. This token will be replaced with the SHA-256
hash of the presented leaf client certificate.

Add a new client_certificate criterion that accepts a "Certificate
Matcher" object. Start with two certificate match conditions:
fingerprint and SPKI hash, each of which can accept either a single
string or an array of strings.

Add new "client-certificate-ok" and "client-certificate-unauthorized"
reason strings.

Update the isValidClientCertificate() method to consider any
client-supplied intermediate certificates. Previously, in order to trust
client certificates issued by an intermediate CA, users would need to
include that intermediate CA's certificate directly in the client_ca
setting. After this change, only the trusted root CA needs to be set: as
long as the client can supply a set of certificates that chain back to
this trusted root, the client's certificate will validate successfully.

Rework the previous CRL checking logic to now consider CRLs for all
issuers in the verified chains.

Add a new max_verify_depth option to the downstream_mtls settings group,
with a default value of 1 (to match the behavior of current Pomerium
releases).

Populate the corresponding setting within Envoy, and also implement a
depth check within isValidClientCertificate() in the authorize service.

Return an error from DownstreamMTLSSettings.validate() if both CA and
CAFile are populated, or if both CRL and CRLFile are populated.

Move the parseCRLs() method from package 'authorize/evaluator' to
'pkg/cryptutil', replacing the existing DecodeCRL() method. This method
will parse all CRLs found in the PEM input, rather than just the first.

(This removes our usage of the deprecated method x509.ParseDERCRL.)

Update this method to return an error if there is non-PEM data found in
the input, to satisfy the existing test that raw DER-encoded CRLs are
not permitted.

Delete the CRLFromBase64() and CRLFromFile() methods, as these are no
longer used.

Add a new match_subject_alt_names option to the downstream_mtls settings
group. This setting can be used to further constrain the allowed client
certificates by requiring that certificates contain a Subject
Alternative Name of a particular type, matching a particular regex.

When set, populate the corresponding match_typed_subject_alt_names
setting within Envoy, and also implement a corresponding check in the
authorize service.