mTLS Support (Client Certificates)

Proposal:
Add optional client certificate and key file configuration when connecting to server over TLS. This is required to connect to an InfluxDB 1.13.0+ server which has client certificate authentication (mTLS) enabled.

Link to preview mTLS documentation for InfluxDB server: https://influxdata.github.io/docs-v2/pr-preview/pr-7523/enterprise_influxdb/v1/administration/configure/security/enable_tls/#enable-mutual-tls-mtls

Current behavior:
Current library does not appear to support client certificates.

Desired behavior:

• Library should support configuring optional client certificate and client private key for TLS connections.
• Client certificate and client key configuration are ignored if TLS is not enabled.
• If only the client certificate is configured and no client key is configured, the library should attempt to load the client certificate as a combined certificate / private key file.
• The client should present the certificate to the server if requested by the server.

Alternatives considered:
There is no alternative if the server is configured to require client certificate authentication (mTLS).

Use case:
mTLS is now supported in InfluxDB 1.13.0+. Customers are beginning to require mTLS in their security policies.

Start by locating the Go client’s TLS connection configuration and its existing TLS-related tests or entry points. Add optional client certificate and key handling, including the combined-file case and TLS-disabled behavior, then verify that the certificate is presented when requested by the server.