feat: allow to use String as span name

[andylokandy]

• Span name and event name now accept both &'static str and String (Into<Cow<'static, str>>), which previously only accept &'static str.
• with_property and with_properties now accept impl Into<Cow<'static, str>>, which previously accept Cow<'static, str>.

[coveralls]

Pull Request Test Coverage Report for Build 6652769436

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 36 unchanged lines in 9 files lost coverage.
• Overall coverage decreased (-0.6%) to 81.716%

---

Files with Coverage Reduction	New Missed Lines	%
minitrace-opentelemetry/src/lib.rs	1	82.09%
minitrace/src/collector/global_collector.rs	2	95.56%
minitrace/src/local/local_span.rs	2	93.44%
minitrace/src/local/span_queue.rs	2	92.68%
minitrace/src/util/mod.rs	2	91.18%
minitrace/src/util/tree.rs	4	97.04%
minitrace/src/local/local_span_stack.rs	6	93.02%
minitrace/src/local/raw_span.rs	8	71.43%
minitrace/src/span.rs	9	94.48%

Totals
Change from base Build 6238061320:	-0.6%
Covered Lines:	1667
Relevant Lines:	2040

---

💛 - Coveralls