Add server_name override for x509_cert plugin

[hackery]

Closes #6809.

As it's my first contrib to this project:

• signed CLA
• make check && make test
• integration test: docker-compose up would not start, problem with docker-credential-secretservice
• add unit test: I couldn't identify a suitable test to add - I could update PR if you have some guidance on that.

[hackery]

Here's the ad hoc test that I've run:

• set up local nginx server with TLS and separate self-signed certs for two virtual hosts, localhost (default) and example.com with CN= to match, no SAN.
• ran x509_cert against localhost without server_name, and then with server_name = example.com
• (not shown here) observed TLS traffic to verify seeing expected SNI in ClientHello message

root@zen:/etc/nginx/tls# sudo openssl req -x509 -newkey rsa:4096 -keyout example.com.key -out example.com.crt -days 365 -nodes -subj '/CN=example.com'

[[inputs.x509_cert]]
  sources = [ "https://localhost:488/" ]
  tls_ca = "/etc/nginx/tls/localhost.crt

./telegraf -config ~/telegraf-conf.d/http.conf  --test
> x509_cert,common_name=localhost,host=zen,issuer_common_name=localhost,public_key_algorithm=RSA,serial_number=27bd5b0513a066e09e23415f65e07605e1ffe855,signature_algorithm=SHA256-RSA,source=https://localhost:488/,verification=valid age=146337i,enddate=1610783209i,expiry=31389662i,startdate=1579247209i,verification_code=0i 1579393546000000000

vs.

[[inputs.x509_cert]]
  sources = [ "https://localhost:488/" ]
  tls_ca = "/etc/nginx/tls/example.com.crt"
  server_name = "example.com"

> x509_cert,common_name=example.com,host=zen,issuer_common_name=example.com,public_key_algorithm=RSA,serial_number=2cb2a733142eb77af2048051573c11f1f7cb6f69,signature_algorithm=SHA256-RSA,source=https://localhost:488/,verification=valid age=468i,enddate=1610929452i,expiry=31535531i,startdate=1579393452i,verification_code=0i 1579393921000000000

Changing server_name = not.example.com (with either CA):

> > x509_cert,common_name=localhost,host=zen,issuer_common_name=localhost,public_key_algorithm=RSA,serial_number=27bd5b0513a066e09e23415f65e07605e1ffe855,signature_algorithm=SHA256-RSA,source=https://localhost:488/,verification=invalid age=147086i,enddate=1610783209i,expiry=31388913i,startdate=1579247209i,verification_code=1i,verification_error="x509: certificate is valid for localhost, not not.example.com" 1579394295000000000