feat: function version migrations

[lkingland]

• 🎁 function version migrations

Introduces the concept of Function versions and associated migrations. Includes a single migration which initializes version and creation fields for Functions <=v0.19.0.

PR #641 takes advantage of function creation timestamp to determine if a Function has been initialized. Functions created prior to this stamp being introduced (forthcoming v0.20.0) should have their Creation stamp initialized to maintain backwards compatibility.

Migrations are applied when a Function is instantiated (read from func.yaml) in the order in which they are registered.

Currently the only migration is that which takes a Function from unversioned to versioned, and initialized a creation timestamp. The version written to func.yaml is the latest migration applied, indicating migrations can be skipped on subsequent loads until a new migration is added to the codebase.

Fixes #674

/kind enhancement

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[codecov]

Codecov Report

Merging #664 (3ab03ec) into main (a0bfc3e) will increase coverage by 0.10%.
The diff coverage is 65.51%.

@@            Coverage Diff             @@
##             main     #664      +/-   ##
==========================================
+ Coverage   37.93%   38.03%   +0.10%     
==========================================
  Files          41       42       +1     
  Lines        3872     3917      +45     
==========================================
+ Hits         1469     1490      +21     
- Misses       2212     2225      +13     
- Partials      191      202      +11     

Impacted Files	Coverage Δ
repository.go	67.74% <ø> (ø)
function.go	54.13% <58.33%> (-6.55%)	⬇️
client.go	61.50% <64.70%> (-1.51%)	⬇️
function_migrations.go	80.00% <80.00%> (ø)
templates.go	78.46% <100.00%> (+0.68%)	⬆️
cmd/root.go	47.66% <0.00%> (+0.93%)	⬆️

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[lance]

This is a nice addition. Thanks!
/lgtm

[lkingland]

Tests have (thankfully, though a bit begrudgingly) uncovered a logical inconsistency in Function instantiation. Marking this as draft until I can get it sorted out.

[zroubalik]

/lgtm

Looking good!