add public API for span links

[beniwohli]

What does this pull request do?

• implement API for providing span links to transactions and spans
• add span link support to OTel Bridge

Related issues

closes #1541

[apmmachine]

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• Start Time: 2022-06-02T06:37:32.279+0000

• Duration: 34 min 46 sec

Test stats 🧪

Test	Results
Failed	0
Passed	4870
Skipped	3455
Total	8325

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (68/68)	💚
Files	100.0% (227/227)	💚
Classes	100.0% (227/227)	💚
Lines	89.513% (17055/19053)	👍 0.064
Conditionals	76.588% (3111/4062)	👍 0.157

[basepi]

Looks good! I originally thought there would need to be changes in opentelemetry/span.py but I double-checked and we're good. Span is a pretty loose wrapper so it doesn't have references to the span links. 👍