Env secret store: add "prefix" metadata option (+ metadata.yaml)

[ItalyPaleAle]

Adds a new option to the Local Env secret store to limit operations to keys with a given prefix.

For example, if the component is initialized with the metadata prefix="MYAPP_", then:

• Get with key FOO will look for MYAPP_FOO. The result will be still returned as FOO (i.e. the prefix name is removed from the returned values)
• GetBulk will only return env vars with the prefix MYAPP_ (which is stripped from the returned value)

Also included:

1. Added metadata.yaml for the component
2. The denylist now denies every secret that starts with DAPR_ (after resolving the prefix), for future-proofing the component.

Tests have been included in the unit tests for the component

Docs issue: dapr/docs#3329