Allow user to burn stables for collateral and governance token

[matthiasmatt]

Description

Include keeper and message to allow user to burn a stable coin in exchange for collateral and governance tokens.

Fixes #54

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

• Message unit test
• Keeper unit test
• Test of burn logic with prifeed integration

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[testinginprod]

overall lookgs good at logic level, some nits only :)