feat(wallets): Move wallets API under customers

[vankiru]

Context

The PR implements api for /customers/:customer_external_id/wallets and /customers/:customer_external_id/wallets/:code/metadata endpoints.

Related PR: getlago/lago-api#4995

Description

The new customer wallets and metadata API looks like this 👇

# GET /customers/:customer_id/wallets
client.customers.wallets.get_all(customer_id)
client.customers.wallets.get_all(customer_id, page: 1, per_page: 100)

# POST /customers/:customer_id/wallets
client.customers.wallets.create(customer_id, params)

# GET /customers/:customer_id/wallets/:wallet_code
client.customers.wallets.get(customer_id, wallet_code)

# PATCH /customers/:customer_id/wallets/:wallet_code
client.customers.wallets.update(customer_id, wallet_code, params)

# DELETE /customers/:customer_id/wallets/:wallet_code
client.customers.wallets.destroy(customer_id, wallet_code)

And for the metadata:

# POST /customers/:customer_id/wallets/:wallet_code/metadata
client.customers.wallets.metadata.replace(customer_id, wallet_code, metadata)

# PATCH /customers/:customer_id/wallets/:wallet_code/metadata
client.customers.wallets.merge(customer_id, wallet_code, metadata)

# DELETE /customers/:customer_id/wallets/:wallet_code/metadata
client.customers.wallets.delete_all(customer_id, wallet_code)

# DELETE /customers/:customer_id/wallets/:wallet_code/metadata/:key
client.customers.wallets.delete_key(customer_id, wallet_code, key)

Nested Resource

The PR also adds a Nested resource that makes it easier to define multi-resource endpoints by introducing an api_resource method that takes the parent resource ids.

Its interface is pretty similar to the existing Resource::Base, with one small difference for update:

• in Nested, all the resource ids come first and the params are the last arguments (if any),
• in Base, it's the other way around.

[groyoh]

Nice job! Integration tests were for wallet but not metadata, is that expected ?