Default branch ID

[blake-regalia]

This PR adds the ability to specify a default branch ID when creating repos.

[dlamoris]

Need to understand more what sysml default branch id is for - is it just literally initializing the branch id and can't be changed later? If it can be changed later then it shouldn't use the mms prefix since we disallow patching those on repo patch, maybe it should be using 'sysmlv2' prefix and the sysml server just uses patch to add the property on the created repo?

Also need to check if any other logic was depending on DEFAULT_BRANCH_ID since the new code makes it so each repo can have a different id than 'master' - should this be something baked into flexo or translated by sysml server

If going ahead will need to migrate existing repos to add that triple

[dlamoris]

seems this is the use of default branch on sysmlv2:
default branch id can be changed - this is basically the fallback branchId if the client doesn't specify a specific branchId for other api that accepts a branchId (ex. post commit)

can just be implemented (from sysmlv2 pov) by using ldp patch on repo with something like sysmlv2:defaultBranch defaultBranchId

on sysmlv2 project creation, if default branch id is specified, can just create that branch from master immediately

If we're to add a flexo default branch semantic - it can only be changed using a 'replace' repo since currently ldp patch does not allow changing things with mms: prefix