[FLINK-32373][sql-client] Support passing headers with SQL Client gateway requests

[afedulov]

What is the purpose of the change

FLINK-32030 and FLINK-32035 enable communication from the SQL Client to the SQL Gateway placed behind a proxy (i.e. a K8S ingress). Such use cases typically require authentication. This PR enables authentication by adding the ability to supply custom headers to the underlying RestClient.

Verifying this change

• Added a test that verifies that custom headers are read and parsed correctly

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• A follow up docs PR will be created to document URLs support in SQL Client gateway mode, including the custom headers feature

[flinkbot]

CI report:

• dc71f24 Azure: SUCCESS

Bot commands

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• @flinkbot run azure re-run the last Azure build

[elkhand]

Do you think it will be not so easy to specify a single env variable value that is separated by newlines? What about considering a different separator for distinguishing a list of header key-value pairs in a single line?

[elkhand]

nit: instead of \n, should we consider platform independent System.lineSeparator()?

[afedulov]

I believe this is actually the best option because, as RFC 7230 states, CRLF within header values is not expected anywhere apart from "obsolete folding", which is not supported anymore.

     header-field   = field-name ":" OWS field-value OWS

     field-name     = token
     field-value    = *( field-content / obs-fold )
     field-content  = field-vchar [ 1*( SP / HTAB ) field-vchar ]
     field-vchar    = VCHAR / obs-text

     obs-fold       = CRLF 1*( SP / HTAB )
                    ; obsolete line folding
                    ; see [Section 3.2.4](https://datatracker.ietf.org/doc/html/rfc7230#section-3.2.4)

field-value, on the other hand, rules out everything from VCHAR. Expecting a delimiter and an escaping strategy for a character that can naturally occur in a cookie is not user-friendly.

[elkhand]

HTTP header-wise it seems ok, I thought it will be challenging for specifying the env variable with multi-line values (each separated with a new line. this is just a nit comment.