[nodes] Add default value to `variablePdaSeedNode`

[lorisleiva]

This could be useful when we know the value of a PDA seed most of the time but would like to allow users to override it when necessary.

For instance, in the following example, we could parameterize the tokenProgram seed whilst making the SPL token the default value.

const node = pdaNode({
    name: 'associatedToken',
    seeds: [
        variablePdaSeedNode('mint', publicKeyTypeNode()),
        constantPdaSeedNode(
            publicKeyTypeNode(),
            publicKeyValueNode('TokenkegQfeZyiNwAJbNbGKPFXCWuBvf9Ss623VQ5DA'),
        ),
        variablePdaSeedNode('owner', publicKeyTypeNode()),
    ],
});