Support URL-encoded values for `OTEL_EXPORTER_OTLP_HEADERS` #4331

Job	Run time
docs	1m 23s
coverage	3m 22s
external-types (opentelemetry)	1m 0s
cargo-deny	24s
external-types (opentelemetry-sdk)	1m 11s
lint	2m 43s
msrv	4m 29s
non-default-examples (opentelemetry-otlp/examples/basic-otlp)	48s
test (stable)	6m 33s
test (beta)	6m 44s
	28m 37s

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