cli: Fix custom provider.cluster

[acheroncrypto]

Problem

provider.cluster serialization is not compatible with its deserialization.

This is because in #2271, a custom deserialization function was implemented to support custom endpoints (as mentioned in #673 (comment)):

anchor/cli/src/config.rs

Lines 593 to 594 in 2ae3774

	#[serde(deserialize_with = "des_cluster")]
	cluster: Cluster,

but there was no custom serialization implementation that was compatible with the new deserialization. Thus, setting provider.cluster to a custom value:

[provider]
cluster = "https://anchor.solana/path"

results in:

Workspace configuration error: Unable to deserialize config: TOML parse error at line 15, column 11
   |
15 | cluster = ["https://anchor.solana/path", "wss://anchor.solana/path"]
   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
invalid type: sequence, expected string or map

when a command, e.g. anchor keys sync, serializes Anchor.toml, as also mentioned in issues such as #3388 and #3222.

Summary of changes

Fix using a custom endpoint in provider.cluster by:

• Implementing a custom serialization function for Cluster.
• Overriding the default serialization of provider.cluster with the new function.

Fixes #3222

[vercel]

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