Add srt_getversion API function

[maxsharabayko]

• Added uint32_t srt_getversion();
• Added description to the API docs
• Added usage in apps

Fixes partially #1016 (does not provide a git commit info)

[jeandube]

Overall this is what I expected get the installed library version number without the need to establish a connection.

[maxsharabayko]

@jeandube @ethouris
A 32-bit integer value still has 8 bits unused: 0x..XXYYZZ
What do you think if we use it for the first 8 bits of git commit ID?

[ethouris]

8 bits gives you 2 hex ciphers. For a git commit ID you need at least 7 hex ciphers. I think we'd better create some more elaborate version id function that can return various things depending on a tag. Might even embrace particular features.