feat: export InfuraRpcEndpoint and AddNetworkCustomRpcEndpointFields types (MetaMask#8764)

## Explanation

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`InfuraRpcEndpoint` and `AddNetworkCustomRpcEndpointFields` were not
exported from `packages/network-controller/index.ts`.

## References

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* Related to MetaMask/metamask-extension#40343

## Checklist

- [ ] I've updated the test suite for new or updated code as appropriate
- [ ] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [ ] I've communicated my changes to consumers by [updating changelogs
for packages I've
changed](https://github.com/MetaMask/core/tree/main/docs/processes/updating-changelogs.md)
- [ ] I've introduced [breaking
changes](https://github.com/MetaMask/core/tree/main/docs/processes/breaking-changes.md)
in this PR and have prepared draft pull requests for clients and
consumer packages to resolve them

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk: this is a public API surface change that only re-exports
existing TypeScript types and updates the changelog, with no runtime
logic changes.
> 
> **Overview**
> **Exposes two previously internal type definitions** from
`@metamask/network-controller` by re-exporting `InfuraRpcEndpoint` and
`AddNetworkCustomRpcEndpointFields` from `src/index.ts`.
> 
> Adds a corresponding `CHANGELOG.md` entry under *Unreleased* noting
the new exports for downstream consumers.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
6074d4f. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: mikesposito

packages/network-controller/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- Add export for `AddNetworkCustomRpcEndpointFields` and `InfuraRpcEndpoint` types ([#8764](https://github.com/MetaMask/core/pull/8764))
+
 ## [31.0.0]
 
 ### Added

packages/network-controller/src/index.ts

@@ -10,8 +10,10 @@ export type {
   NetworkState,
   BlockTrackerProxy,
   ProviderProxy,
+  AddNetworkCustomRpcEndpointFields,
   AddNetworkFields,
   UpdateNetworkFields,
+  InfuraRpcEndpoint,
   NetworkControllerStateChangeEvent,
   NetworkControllerNetworkWillChangeEvent,
   NetworkControllerNetworkDidChangeEvent,