[feature] Web3Modal: Add custom headers to explorer-api request

[ignaciosantise]

What problem does this new feature solve?

Adds user-agent and referer headers in explorer-api requests

Describe the solution you'd like

• user-agent that follows the pattern <web3modal version>/<core sdk version>/<operating system>
• referer containing the name provided in the sdk metadata (with no spaces)

example:

headers: {
 user-agent: 'w3m-ios-1.0.0/swift-1.5.7/ios-16.5',
 referer: 'Showcase'
}