feat(kms): do not require distributed locking when only using one kms key

[0xfourzerofour]

Closes #361

Proposed Changes

• Change create conditional signer based on length of kms keys so that the kms_guard can be turned into an optional field and not created with only one key id

[codecov]

Codecov Report

Merging #475 (ae65be4) into main (dc7a26c) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

Files	Coverage Δ
crates/builder/src/signer/aws.rs	0.00% <0.00%> (ø)

Flag	Coverage Δ
unit-tests	52.60% <0.00%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
rundler binary	0.00% <ø> (ø)
builder	47.67% <0.00%> (-0.15%)	⬇️
dev	0.00% <ø> (ø)
pool	59.92% <ø> (ø)
provider	1.33% <ø> (ø)
rpc	18.26% <ø> (ø)
sim	82.20% <ø> (ø)
tasks	∅ <ø> (∅)
types	93.56% <ø> (ø)
utils	7.20% <ø> (ø)