Pass origin HTTP header on requests to Infura

[bdresser]

Adding a MetaMask-specific header to Infura requests will let us segment our traffic and understand more about daily usage.

This header used to exist but was pulled because it caused issues with custom RPC options. We should only include the header when we're certain the request is headed to Infura. See #2250 and maybe #2138.

Prior implementation used x-metamask-origin – we should check with Infura team to see if this is suitable, or the simple origin header will do, or if there's something else they prefer

[bdresser]

relates to #4279

[bdresser]

@danfinlay @kumavis any concerns with using x-metamask-origin and passing through the dapp that initiated the transaction? This should let us see total numbers from MetaMask as well as generate a list of dapps folks are using (beyond what Infura is able to look up by contract)

[danfinlay]

I think that's fine, I believe that's what we used previously. I would want @kumavis to chime in on whether this is going to trigger a preflight.

[whymarrh]

As an FYI, the X- prefix for custom headers is deprecated (RFC 6648) so if possible we might want to talk to Infura about their desired custom header (e.g. maybe a Infrua-Source: MetaMask/<dapp name> or something).

[whymarrh]

Did we hear back from Infura about which header they'd like us to implement?

[bdresser]

implemented in #6023