Export function that signs advertisements

[masih]

Export the function that is used to sign advertisements so that it can
be reused for testing purposes in index-provider.

Note that its counterpart verify function is already exported.

Bump version in preparation for release.

[github-actions]

Suggested version: v0.3.2
Comparing to: v0.3.1 (diff)

Changes in go.mod file(s):

(empty)

gorelease says:

# github.com/filecoin-project/storetheindex/api/v0/ingest/schema
## compatible changes
SignAdvertisement: added

# summary
Suggested version: v0.4.0

gocompat says:

(empty)

[codecov-commenter]

Codecov Report

Merging #217 (dbc1835) into main (872e864) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #217      +/-   ##
==========================================
+ Coverage   28.90%   28.96%   +0.06%     
==========================================
  Files          84       84              
  Lines       11645    11645              
==========================================
+ Hits         3366     3373       +7     
+ Misses       7770     7765       -5     
+ Partials      509      507       -2     

Impacted Files	Coverage Δ
api/v0/ingest/schema/envelope.go	67.27% <100.00%> (ø)
api/v0/ingest/schema/utils.go	65.00% <100.00%> (ø)
internal/ingest/linksystem.go	61.64% <0.00%> (+0.17%)	⬆️
internal/ingest/cidwaiter.go	77.96% <0.00%> (+10.16%)	⬆️

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[masih]

CI test failures seem unrelated and down to intermittently failing tests it seems.
merging.