Minor refactoring of transactional coordinator code

[mmaslankaprv]

Cleaned up some code in transaction manager area including:

• reduced number of transactional id copies for lookups
• aligned transaction state print format with rest of the code
• using strong named_type in model::producer_identity

Backports Required

• [ ] none - not a bug fix
• [ ] none - this is a backport
• [ ] none - issue does not exist in previous branches
• [ ] none - papercut/not impactful enough to backport
• [ ] v24.1.x
• [ ] v23.3.x
• [ ] v23.2.x

Release Notes

• none

[mmaslankaprv]

unrelated failure in: gtest_raft_rpunit