The MASP conversions query now requires a MASP epoch to be specified. (backport #4776)

[mergify]

Describe your changes

Querying the conversion state becomes more difficult as nodes progress to higher and higher epochs. For instance, on a mainnet clone running on accelerated epochs, client queries for conversions always timeout (even after modifying timeout_broadcast_tx_commit) when the node goes beyond a certain MASP epoch. This happens because the conversion state grows linearly with the MASP epoch counter. This PR attempts to address this problem by making the conversions RPC endpoint require that clients specify the MASP epoch they want conversions from. This implies two things: first the size of the response from the conversions RPC endpoint should now be constant (equal to the number of tokens in the shielded rewards program), and second a client requiring all conversions now has to do a separate query for each MASP epoch.

Checklist before merging

• If this PR has some consensus breaking changes, I added the corresponding breaking:: labels
  • This will require 2 reviewers to approve the changes
• If this PR requires changes to the docs or specs, a corresponding PR is opened in the namada-docs repo
  • Relevant PR if applies:
• If this PR affects services such as namada-indexer or namada-masp-indexer, a corresponding PR is opened in that repo
  • Relevant PR if applies:

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This is an automatic backport of pull request #4776 done by [Mergify](https://mergify.com).

[mergify]

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