Add ability for the opamp client to use the x-sf-token header

[breedx-splk]

If the opamp client is to be used without a collector, then it will need the ability to set the token header.

[robsunday]

Are you going to add OpampClientConfigurationFactory and OpampClientConfigurationFactoryTest changes in this PR, or another one will be created?

[breedx-splk]

Are you going to add OpampClientConfigurationFactory and OpampClientConfigurationFactoryTest changes in this PR, or another one will be created?

Why, does OpampClientConfigurationFactory require something extra/special? I thought this would already cover those.

[robsunday]

OpampClientConfiguration.Builder.withAccessToken() is called in tests to setup confiuration so tests pass, but it is not called from production code when starting the agent.

OpampClientConfiguration is created in two methods:

• OpampClientConfigurationFactory.createConfigurationFromDeclarativeConfig
• OpampClientConfigurationFactory.createConfiguration(ConfigProperties) - used for environment config

OpampClientConfiguration.Builder.withAccessToken() should be called there as well.

[robsunday]

We should either use constants in all places here, or just a plain strings. This constant is not used anywhere else, so to me it is not worth making this class bigger by adding a constant.

[breedx-splk]

That constant is used in a few other places. I prefer using existing constants over duplicating string literals, which is prone to creating inconsistencies. I didn't create constants for the OTHER string key literals because they are only used here.