Respect original content when creating secrets (#24745)
Fix #24721.
Follow what GitHub does:
- Don't trim spaces for secrets.
- Newline should be `\n` instead of `\r\n`.
Did some tests with:
```yaml
name: secrets
on: push
jobs:
show_secrets:
runs-on: ubuntu-latest
steps:
- name: Dump secrets context
run: echo '${{ toJSON(secrets) }}' | base64
```
`AAAAAA`:
```text
AAAAAA
AAAAAA
```
`BBBBBB`:
```text
BBBBBB
BBBBBB
```
On GitHub:
<img width="675" alt="image"
src="https://github.com/go-gitea/gitea/assets/9418365/0ec60652-c2a3-47bb-9f9d-7e81665355a8">
On Gitea (before):
<img width="673" alt="image"
src="https://github.com/go-gitea/gitea/assets/9418365/cce818bf-5edc-4656-86e1-2c81c304cdb2">
On Gitea (after):
<img width="673" alt="image"
src="https://github.com/go-gitea/gitea/assets/9418365/0b3b15af-4d48-4bab-a334-4738a1b0eb4a">
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