Support URL-encoded values for `OTEL_EXPORTER_OTLP_HEADERS` #4332

Job	Run time
docs	1m 27s
lint	1m 33s
external-types (opentelemetry)	1m 1s
cargo-deny	24s
external-types (opentelemetry-sdk)	1m 11s
msrv	4m 34s
non-default-examples (opentelemetry-otlp/examples/basic-otlp)	48s
test (stable)	6m 29s
coverage	3m 34s
test (beta)	6m 45s
	27m 46s

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