Support custom HTTP headers

[kwSeo]

In addition to Tenant header, we need to add custom HTTP header when RemoteWrite request.
How about providing the feature to add custom HTTP Headers like below?

Flag Name

--remote-custom-header

Example

avalanche --remote-url "http://host/prom/push" \
        --remote-tenant "fake" \
        --remote-tenant-header "x-my-tenant" \
        --remote-custom-header "x-custom-header: test1" \
        --remote-custom-header "Authorization: Bearer abcdefg"