Add self custody available field to get wallet endpoint

[clD11]

Summary

This PR adds the self custody available field to get wallet endpoint.

The endpoint should the below response with the new selfCustodyAvailable field.

{
  "paymentId": "af5f35a5-1809-4e73-91c4-59b8dbe3a42f",
  "walletProvider": {
    "id": "",
    "name": "brave"
  },
  "altcurrency": "BAT",
  "publicKey": "485c991e35fea0a174786eb9062a76ba6aa7c8bedf1d438a200205475f73db4f",
  "selfCustodyAvailable": {
    "solana": true
  }
}

this is a map of providers, currently we only have one Solana.

Please see the spec in the linked issue for more detail.

resolves 168

Type of Change

• Product feature
• Bug fix
• Performance improvement
• Refactor
• Other

Tested Environments

• Development
• Staging
• Production

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Manual Test Plan

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