GenAI: define and document declarative configuration options

[lmolkova]

We can define configuration properties formally - this is used by declarative config.

We can also document them (in plain text for now) similarly to

semantic-conventions/model/http/registry.yaml

Lines 97 to 104 in c3485c7

	If the HTTP instrumentation could end up converting valid HTTP request methods to `_OTHER`, then it MUST provide a way to override
	the list of known HTTP methods. If this override is done via environment variable, then the environment variable MUST be named
	OTEL_INSTRUMENTATION_HTTP_KNOWN_METHODS and support a comma-separated list of case-sensitive known HTTP methods.
	
	If this override is done via declarative configuration, then the list MUST be configurable via the `known_methods` property
	(an array of case-sensitive strings with minimum items 0) under `.instrumentation/development.general.http.client` and/or
	`.instrumentation/development.general.http.server`.

Some common properties to define:

• opt-in to bleeding edge is controlled with exiting ExperimentalSemconvConfig and OTEL_SEMCONV_STABILITY_OPT_IN
• capture content
• record spans and/or events

And maybe others in https://github.com/open-telemetry/opentelemetry-python-contrib/blob/main/util/opentelemetry-util-genai/src/opentelemetry/util/genai/environment_variables.py