Support unix socket for otlp exporter

[KimMJ]

Is your feature request related to a problem? Please describe.
I'm trying to use uds to export otel signals. But it was failed by endpoint validation.
Currently opentelemetry-java have protocol validation which allows only http and https prefix.

Describe the solution you'd like
Make opentelemetry-java export otel signals through unix socket

Describe alternatives you've considered
none

Additional context

opentelemetry-java/exporters/otlp/all/src/main/java/io/opentelemetry/exporter/otlp/internal/OtlpConfigUtil.java

Line 188 in a839ef5

"OTLP endpoint scheme must be http or https: " + endpointUrl.getProtocol());

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[jack-berg]

This is a reasonable request. This spec document lays out the required configuration options for SDK implementations of OTLP exporters, and doesn't explicitly mention unix sockets. But the document is meant to establish minimum common configuration, not to limit configuration to only those options.

[KimMJ]

open-telemetry/opentelemetry-collector#11941

looks opentelemetry-go and otel collector already supports unix sockets

[KimMJ]

open-telemetry/opentelemetry-specification#4404
it is not defined in spec, though some sdk supports it