Add ETag support to both server and client

Is your feature request related to a problem? Please describe.
The ETag is not returned from the server's API. There is no configuration to enable it.
The client doesn't make use of ETag at all.

Describe the solution you'd like
The ETag header can be easily added to the response header using the ShallowEtagHeaderFilter of Spring Web. This calculates the value from the response body. In case of GIT being configured as the environment repository, the ETag can be set to the value of the commit ID of the given label. This would simplify the ETag calculation (i.e. doesn't need to resolve all files) and would add the possibility to return ETag in the HEAD response body.

The client can then consider the ETag to figure out if it needs to fetch the configuration from the server again. This is useful in the configuration reload use cases.

Describe alternatives you've considered
none

Additional context
n/a

Start by tracing the Spring Web server response path and the client configuration reload path; review how the Git environment repository exposes the commit ID and how HEAD responses are handled. Done means the server can return ETags, including the Git commit ID where applicable, and the client uses them to avoid unnecessary configuration fetches.