Python-v2: paywall builder

[phdargen]

Description

• Fixes paywall UI that wasn't rendered, similar to 5939a78 for go
• Adds new paywall builder pattern similar to TS paywall provider for more customization:

from x402.http.paywall import create_paywall, evm_paywall, svm_paywall
paywall = (
    create_paywall()
    .with_network(evm_paywall)
    .with_network(svm_paywall)
    .with_config(app_name="x402 Paywall Demo", testnet=True)
    .build()
)
app.add_middleware(
    PaymentMiddlewareASGI,
    routes=routes,
    server=server,
    paywall_provider=paywall,
)

Tests

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits

[vercel]

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