[receiver/snmpreceiver] Add boilerplate skeleton for SNMP metric receiver

[StefanKurek]

Description:
This is the boilerplate setup of the SNMP Receiver. It collects without erroring, but does not have any useful config or actually collect any SNMP data at this point.

Link to tracking Issue:
13409

Testing:
No unit or integration tests. Just manually ran to see if it worked.

Documentation:
README added for what the SNMP receiver will ultimately be.

[djaglowski]

Looks good. Just two doc nits

[djaglowski]

Suggested change

	[beta]:https://github.com/open-telemetry/opentelemetry-collector#in-development
	[in development]:https://github.com/open-telemetry/opentelemetry-collector#in-development

[djaglowski]

Suggested change

	| Stability | [in-development] |
	| Stability | [in development] |

[djaglowski]

Looks like it needs a make gotidy as well

[djaglowski]

It appears that internal/components/TestDefaultReceivers is failing:

=== RUN   TestDefaultReceivers
    receivers_test.go:358: 
        	Error Trace:	/home/runner/work/opentelemetry-collector-contrib/opentelemetry-collector-contrib/internal/components/receivers_test.go:358
        	Error:      	"[{active_directory_ds %!s(bool=true) %!s(components.getReceiverConfigFn=<nil>)} {aerospike %!s(bool=false) %!s(components.getReceiverConfigFn=<nil>)} {apache %!s(bool=false) %!s(components.getReceiverConfigFn=<nil>)} ...]" should have 73 item(s), but has 72
        	Test:       	TestDefaultReceivers
        	Messages:   	All receivers must be added to the lifecycle suite

[djaglowski]

Link to failure: https://github.com/open-telemetry/opentelemetry-collector-contrib/actions/runs/3191785503/jobs/5208530885#step:8:9653

[StefanKurek]

Link to failure: https://github.com/open-telemetry/opentelemetry-collector-contrib/actions/runs/3191785503/jobs/5208530885#step:8:9653

@djaglowski ok, so the list of tests was always cut off for me. i had to expand/collapse them a couple times to get everything to show up.