feat(line): define line pieces interface

[Ovilia]

Brief Information

This pull request is in the type of:

• bug fixing
• new feature
• others

What does this PR do?

Line style for varied segments and connectNulls

Fixed issues

#17138

Details

Before: What was the problem?

After: How does it behave after the fixing?

Document Info

One of the following should be checked.

• This PR doesn't relate to document changes
• The document should be updated later
• The document changes have been made in apache/echarts-doc#xxx

Misc

ZRender Changes

• This PR depends on ZRender changes (ecomfe/zrender#xxx).

Related test cases or examples to use the new APIs

N.A.

Others

Merging options

• Please squash the commits into a single one when merging.

Other information

[echarts-bot]

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⚠️ MISSING DOCUMENT INFO: Please make sure one of the document options are checked in this PR's description. Search "Document Info" in the description of this PR. This should be done either by the author or the reviewers of the PR.

[github-actions]

The changes brought by this PR can be previewed at: https://echarts.apache.org/examples/editor?version=PR-20497@a45b824