[MAINT] Make the SRT header file and all interface features independent on compile options

[ethouris]

The srt.h header file should not contain any conditionals referring to the optional features of SRT.

The header may contain conditional macros that enable any new experimental or preview features. These are things that are needed to be conditionally-blocked because:

• The feature is not yet officially supported and it should not be available for default users
• This isn't the part of the official API and as such it may undergo changes before it becomes one

Just all conditional macros that refer to any optional features that can be enabled or disabled at build time - regardless if they are enabled or disabled in the default build configuration - should be removed. All parts of the SRT library interface must be available if they are part of the official API release (that is, they are not experimental/preview) and the application using that part of the API must be still able to be compiled and linked. Just the use of this part of the API shall result in a runtime error.

Possible works to be done for this, except for removal all conditionals unrelated to expreimental/preview features:

• Change implementation files to allow proper compiling and linking even if the application uses non-supported features
• Ensure that the disabled features generated proper error codes
• CONSIDER: Might be a specific error type designed for situation described as "not enabled at compile time", if no other is appropriate

Another thing to be considered: There should be at best no specific naming of experimental and preview features. There better be only a single enabler, or just two named ENABLE_EXPERIMENTAL and ENABLE_PREVIEW, which will enable all features that are currently qualified this way. The intent for these should be:

• ENABLE_PREVIEW: for features that do not change any potential stability of the rest of the library features if it is not currently in use. There might be potential stability problems only if this feature is in use, and the API may change in the next version.
• ENABLE_EXPERIMENTAL: for features which's impact on the existing features of SRT is unknown, and otherwise like above.