chore: add private send transfer function

[colin-axner]

Description

Splitting #1918 into two steps:

• add private send transfer function (will be backported)
• remove SendTransfer public API/migrate tests to MsgTransfer

By backporting the private send transfer function, API additions can be done to MsgTransfer without breaking API

ref: #1918

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[colin-axner]

This will need to be backported only to new minor releases (v2.5, v3.4, v4.2, v5.1), which we don't have labels for yet

[codecov-commenter]

Codecov Report

Merging #2425 (ee2c480) into main (3c21b5e) will increase coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2425      +/-   ##
==========================================
+ Coverage   78.59%   78.61%   +0.01%     
==========================================
  Files         178      178              
  Lines       12322    12333      +11     
==========================================
+ Hits         9684     9695      +11     
  Misses       2206     2206              
  Partials      432      432              

Impacted Files	Coverage Δ
modules/apps/transfer/keeper/msg_server.go	100.00% <100.00%> (ø)
modules/apps/transfer/keeper/relay.go	88.66% <100.00%> (+0.43%)	⬆️

[chatton]

LGTM